I had a similar problem. After shooting ~840 photos in that particular session (a high school sporting event) on a SanDisk Extreme 32GB SD card, subsequent photos would show up on the back of the camera when I shot them, but would not be accessible from the review mode and were apparently not being saved to the card. I swapped the two cards and continued shooting without problems, but lost about 100 photos before I discovered this.
I'm formatting the cards to see if this fixes the problem. I haven't yet tried to duplicate it.